Rename "master" branch to "main"

[gclough]

The current practice is to use main for the main branch, instead of master... so we should swap, to keep up-to-date:

https://www.linkedin.com/pulse/technology-notes-how-tos-infrastructure-git-master-main-eldritch/

[natedennis]

or you could leave it alone and be stable instead of trendy

[otakup0pe]

Strongly agree with renaming the main branch to something other than master. Only concern is whether this will interfere with how galaxy works.